FIGS. 3 and 4 show a conventional center differential used in a 4-wheel drive vehicle. The terms "front" and "rear" as used herein shall mean and refer to the respective forward and rearward directions of the vehicle body.
As shown in the drawings, a ring gear 51 is disposed around a differential case 52 and connected to a drive pinion (not shown). Front and rear side gears 53 and 54 are disposed within the differential case 52 and are splined to a drive shaft (not shown). The side gears 53 and 54 are provided with helical gear teeth 55 at their outer surfaces.
Four sets of pinion gear assemblies 57 each having first and second pinion gears 60 and 61 are disposed within gear grooves 56 formed on an inner wall of the differential case 52 and are engaged with the side gears 53 and 54. The first pinion gear 60 has long and short helical gears 58 and 59 disposed along a common axis, and the second pinion gear 61 has long and short helical gears 63 and 62 disposed along a common axis. The first and second pinion gears 60 and 61 of each set of pinion gear assemblies 57 are disposed such that the long and short helical gears 58 and 59 mesh with the short and long helical gears 62 and 63 of the second pinion gear 61, respectively. In addition, the long helical gears 58 and 63 mesh with the rear and front side gears 54 and 53, respectively.
End washers 65 are disposed between the side gears 53 and 54 and the differential case 52.
When the vehicle runs straight forward, because no rotation speed difference occurs between the front and rear side gears 53, and 54, the pinion gears 60 and 61 do not rotate about their axes and the front and rear side gears 53 and 54 rotate together with the differential case 52.
When a speed difference occurs between the front and rear side gears 53 and 54, the pinion gears 60 and 61 compensate for the speed difference by reversibly rotating with respect to each other.
When a load difference occurs between the front and rear wheels, the front and rear side gears 53 and 54 operate such that a mesh-reacting force is generated between the side gears 53 and 54 and the pinion gears 60 and 61. The mesh-reacting force acts as a thrust force applied to a teeth surface of the helical gear in an axial direction.
Accordingly, the side gears 53 and 54 are forced against the washers 64 and 65, and the pinion gears 60 and 61 are engaged with the differential case 52 by the thrust force. As a result, the side gears 53 and 54 and the pinion gears 60 and 61 are integrally connected to the differential case 52, thereby distributing equal drive torque to front and rear differentials (not shown) to realize limited slip differential operation.
However, in the above-described differential, because the diameters of the front and rear side gears are identical to each other and the pinion gears are meshed with the front and rear side gears in parallel, a friction area between the pinion gears and the inner wall of the differential case is limited, making it difficult to obtain limited slip differential force during a limited slip differential mode.
In addition, it is preferable to differently distribute drive torque to front and rear wheels to meet a balance with respect to the ground contacting force of the front and rear wheels. However, in the conventional center differential, because the distributing ratio of drive torque to the front and rear wheels is fixed at 50:50, it is difficult to enlarge the range of driving capability of the center differential.